The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to a rechargeable alkali metal electrochemical cell, particularly a lithium-ion secondary cell.
Lithium secondary cells have been under development for many years. Early efforts focused on the use of a lithium anode coupled with metal oxide and metal sulfide cathode materials such as manganese dioxide, titanium disulfide, and others. Despite the enormous amount of research performed on lithium secondary systems, cells with metallic lithium anodes have not found widespread commercial use. Of concern are the inherent safety problems associated with them. During use, lithium plating can occur in an undesirable manner with dendritic lithium penetrating through the separator and short circuiting the cell. In addition to rendering the cell inoperative, this condition can cause the cell to vent or, in extreme cases, to explode.
During the past decade, increased attention has focused on the use of electrode materials which are capable of more effectively intercalating and de-intercalating lithium ions than the previously used metal oxides and metal sulfides. Cells incorporating such second generation electrode materials are typically referred to as lithium-ion or lithium-rocking chair systems. Although the energy density of these secondary cells is lower than that of primary cells containing lithium metal anodes, they exhibit a higher open circuit voltage, an acceptably high operating voltage and, in many cases, equivalent or better rate capability than many previously developed lithium secondary systems. Most importantly, their safety is generally accepted to be much better.
Presently, lithium-ion secondary cells are used in a large number of commercial applications including telephones, camcorders and other portable electronic equipment. They have been made in a variety of shapes, sizes and configurations including coin, button, cylindrical and prismatic cells. There are several other applications, however, for which rechargeable lithium cells and, in particular, lithium-ion secondary cells may be used but for which present day constructions are unsuitable. Such applications include medical instruments, implantable medical devices and surgical tools.
For many of these applications, the use of prior art lithium-ion secondary cells is unacceptable due to their shape and construction. In certain types of medical applications, prismatic cells which are sized and shaped for use within the human body are most preferred.
U.S. Pat. No. 3,169,889 to Zahn shows a storage cell having electroplates and a separator that are xe2x80x9cscallopedxe2x80x9d so as to conform to the cell profile when the assembly is folded (see FIGS. 1 and 2 of the Zahn patent).
U.S. Pat. No. 3,856,575 to Hughes relates to an electric cell with electroplates that are appropriately shaped and spaced to be accommodated in cylindrical casings, but not wound therein.
U.S. Pat. No. 2,422,045 to Ruben is representative of wound structures in batteries (see FIGS. 3 and 13 of the Ruben patent).
None of these patents disclose a wound cell structure for a lithium-ion secondary cell.
Accordingly, there exists the need for lithium-ion secondary cells which are, among other things, spirally wound and have a shape suitable for use with implantable medical devices.
The present invention meets the above-described need by providing a high energy density lithium ion secondary cell having an irregular shape. The secondary electrochemical cell includes a negative electrode comprising a negative electrode active material which intercalates with an alkali metal, and a positive electrode comprising a positive electrode active material which intercalates with the alkali metal. The negative electrode and the positive electrode are electrochemically associated with each other and housed in an irregular-shaped casing. The electrodes are disposed such that a periphery of the positive electrode is completely bounded by a periphery of the negative electrode to prevent alkali metal from plating as the cell is repeatedly cycled between a charged and a discharged condition. An electrolyte solution activates the negative and positive electrodes. The cell includes unitary anode and cathode electrodes having an irregular shape that are spirally wound or folded with a suitable separator to form a lithium-ion secondary cell that is capable of use in an implantable biomedical device.